1. Field of the Invention
This invention relates to a superconductive ceramics laminate comprising bismuth (Bi), strontium (Sr), calcium (Ca), copper (Cu) and oxygen and, more particularly, a superconductive ceramics laminate based on Bi--Sr--Ca--Cu--O which is formed with a film of composite oxide crystals oriented in a specific direction and to a method for producing such a laminate.
2. Statement of the Prior Art
Superconductors are materials showing the property (superconduction) of reducing their electrical resistance to zero under such conditions as defined by values equal to or lower than critical values represented by the critical temperature Tc, critical magnetic field Hc and critical current density Jc.
Known as oxide ceramics showing superconductivity at about 90K is a composite oxide based on Y--Ba--Cu--O. Recently, another superconductive ceramic based on Bi--Sr--Ca--Cu--O has been discovered as a composite oxide having a Tc of 80 to 110K. Such Bi--Sr--Ca--Cu--O systems are more stable and resistant to external environments such as moisture than the Y--Ba--Cu--O systems.
Ordinarily, such composite oxide ceramics is obtained by sintering a compact of starting ceramics powders, under pressure or on an atmosphere.
Nb--Ti or Nb.sub.3 Sn alloys have been known as the superconductive materials practically used in the form of wires, tapes and coils. However, these materials have to be cooled to the temperature of liquid helium at considerable expense.
Various proposals have thus been made to put to practical use wire and tapes using high-temperature superconductors comprising composite oxides having high critical temperatures.
In order to put superconductors to practical use, it is necessary that they can be formed into wires and tapes and that they have a large critical current density, Jc. Although not clarified, the crystal structure of the superconductive ceramics based on Bi--Sr--Ca--Cu--O, as proposed, is characterized in that, of the crystal axes, the c-axis is much longer in length, say, five times or more as long as the a- or b-axis, and the atoms are arranged and stratified on the ab-plane. In a superconductive state, therefore, it is theoretically considered that electrons migrate along the ab-plane (perpendicular to the c-axis), but hardly move along the c-axis.
In wires and coils using conventional high-temperature superconductive ceramics, little attention is paid to such a crystal structure. Nor is any practically high current density obtained without providing means for passing large currents through wires and tapes in their longitudinal direction.
Vapor-phase techniques such as vapor deposition, sputtering and CVD have been proposed as the processes for orienting crystals as mentioned above to pass large currents through wires and tapes in their lengthwise direction. However, problems with these techniques are that films are produced through complicated steps at low rates and very high costs, thus making it difficult to process them into continuous forms such as wires and tapes.
An object of the present invention is to provide a superconductive ceramics laminate showing a critical current density so practically high that when the laminate is formed into a wire or tape, a large current can be passed therethrough in the lengthwise direction. Another object of the invention is to provide a method for making superconductive ceramics laminates of satisfactory crystal orientation in a simple and inexpensive manner.
There is also a need in this field for heat treatments at high temperatures, which leads to the necessity of using heat-resistant materials such as heat-resistant alloys as substrates. When ceramics films are formed directly on substrates of heat-resistant alloys, however, difficulty is encountered in obtaining oriented films due to the poor coherency in terms of the lattice constants therebetween. Further, the superconductive phases of ceramics fail due to a reaction occurring between the ceramics and the substrates during a heat treatment.
Thus, a further object of the present invention is to provide a superconductive ceramics laminate which shows a critical current density so practically high that when it is formed into a wire or tape, a large current can be passed therethrough in its longitudinal direction, and for which various heat-resistant materials can be used as the substrate, and to provide a method for making superconductive ceramics laminates of satisfactory crystal orientation in a simple and inexpensive manner.